This multi-center (Dent Neurologic Institute, Bflo., NY; University of Rochester Medical Center, Roch., NY; Walter Reed Army Medical Center, Wash., DC) two year clinical trial intends to definitively ascertain whether or not human fibroblast interferon (IFN-B) administered intrathecally (IT) quantifiably reduces exacerbations in patients with MS. Eighty MS patients with exacerbation rates of .66 exacerbations per year, or greater, will be randomly assigned (stratified by exacerbation rates) to groups of 40 recipients and 40 controls. Recipients will receive IFN IT for 24 weeks (twice in week 1, once per week during weeks 2, 3, and 4, and then once every 4 weeks from week 5 to 24). Controls will undergo false lumbar punctures (LPs) with sterile water being injected subcutaneously according to the same schedule. True and false LPs will be performed by a treating physician. Serial neurologic examinations will be performed by a separate examining physician who will determine if the patients experience exacerbations while on the study and also quantify their functional group and disability status scores. Data regarding occurrence of exacerbations and clinical status will be transmitted to the statistics center where patients' pre study and on study exacerbation rates and clinical conditions will be compared. Response will primarily be defined by change or lack of change in exacerbation rates (simple t test on difference in changes in rates) and secondarily by changes in clinical condition and laboratory parameters. The patients and examining physicians will be blinded. Serial laboratory tests to be performed that might provide quantifiable data include: NMR, CT, VER, CSF (IgG, MBP, oligoclonal bands, IFN levels) and serum IFN levels. If the efficacy of IT IFN in reducing exacerbations (suggested in preliminary study) is confirmed, then this method of treatment could be used on a widespread basis in MS patients soon after certain diagnosis to prevent major disability by reducing or eliminating exacerbations.